[unreadable] We will develop a new high efficiency silicon x-ray detector, with good energy resolution and high count rate capability for synchrotron x-ray fluorescence (XRF) applications. We propose to extend the very successful silicon drift detector design, currently fabricated on thin 0.3 mm thick substrates, into the realm of thick detectors (up to 1.5 mm thick). This added thickness is critical in extending the x-ray absorption capability of the detectors up to 40 keV. The current 0.3 mm thick silicon rapidly falls off in efficiency above 10 keV (down to 9% at 30 keV), whereas 1.5 mm is 42 % efficient at 30 keV. Synchrotron XRF techniques are becoming essential tools in the field of bio-medical applications. Several synchrotrons - such as the Stanford Synchrotron Radiation Laboratory and the National Synchrotron Light Source - have hard x-ray beam lines that are dedicated to XRF analysis. Third generation synchrotrons, such as the Advanced Photon Source (APS), have photon flux brilliances in the 6-40 keV region that allow efficient x-ray fluorescence analysis with probe sizes of a micrometer and below. All of these beamlines require high count rate, good energy resolution energy dispersive x-ray detectors that have efficient absorption up to 40 keV. The new, thicker drift detectors will have the advantages of non-cryogenic cooling, good energy resolution, and an order of magnitude improvement in count rates compared with conventional silicon and germanium XRF detectors, while providing a significant increase in efficiency above 10 keV. [unreadable] [unreadable] The Phase I work will include the design and fabrication of the thick silicon drift detectors, and the characterization of the detectors with respect to noise, energy resolution, efficiency and count rate capability in response to x-rays in the 5-60 keV range. The Phase II work will include development of a thick drift detector array, and the associated low noise readout electronics, for the construction of an XRF spectrometer that will be evaluated in a synchrotron XRF experiment at the APS at Argonne National Laboratory. [unreadable] [unreadable]